The present invention relates generally to thermometers and more particularly thermometers designed to withstand mechanical abuse and excessive temperatures which might otherwise cause damaging stress. The present invention is particularly useful in connection with the protection of thermometers utilizing bimetallic temperature responsive elements and rotating temperature indicators.
It is well recognized that thermometers, including particularly thermometers utilizing bimetallic temperature responsive elements, are very fragile devices, subject to damage from rough handling and excessive vibration. While the fragility of these thermometers is well known, economic and other considerations often require that they be used in applications from which rough handling and vibration cannot be entirely eliminated. In the automobile industry, for example, dial thermometers having bimetallic temperature responsive elements and rotary temperature indicators are mounted in exterior rear view mirrors of luxury cars. Unfortunately, abuse by production workers on the automotive assembly line can result in the mounting of miscalibrated or broken thermometers.
Since the damage of such dial thermometers generally results from uncontrolled rotary oscillation of the thermometer indicator and attached components, one solution employed to reduce this mechanically induced damage has been to provide a mechanical stop which limits oscillatory movement of the indicator beyond a pre-determined point. This remedy, however, has been found to introduce equally undesirable temperature induced damage when the thermometer must be subjected to extreme temperatures.
Efficient automobile production requires that externally mounted thermometers be attached prior to touch up of the automobile exterior. Thus, an automobile with thermometer in place is moved through a touch up oven which generally is maintained at temperatures in excess of 200.degree. F. As the bimetallic element of the thermometer responds to the temperature of the touch up oven and rotates the dial, the indicator stop will engage while the bimetallic element continues to apply an increasing torque. This torque can reach levels sufficient to break weldments between the bimetallic element and the shaft supporting the indicator. Even where weldments are not broken, these torque stresses are likely to permanently distort the temperature responsive element of to displace the shaft between the bimetallic element and the dial from its mounting in the dial, thereby destroying thermometer calibration.